Producers of LCD screens supply these screens to users who will integrate them into an end product, such as an instrument board or a portable device for example. The screens are supplied encapsulated in a rigid mechanical structure forming a frame.
FIG. 1 illustrates an example of rear encapsulation of an LCD screen. The example represented corresponds to a basic configuration comprising a frame 10 clenching an LCD cell, that is to say a plate 1 and a backplate 2 assembled in a leaktight manner so as to form a cavity containing the liquid crystal XL, between the two plates, and two polarizers assembled (or more exactly laminated), each by gluing: a front polarizer 3 laminated on the backplate 2 and a rear polarizer 4 laminated on the plate 1.
The plate 1 is larger than the backplate 2, thus offering a peripheral zone 1a overhanging the backplate. This peripheral zone is clamped between an outer frame 10.a and an inner frame 10.b which form the mechanical structure 10 for encapsulating the screen. At the front face A there is usually a seal j between the outer frame and the front face of the plate 1, in the peripheral zone 1a. At the rear face B it is ordinarily a spacer 12 which is used between the inner frame 10.b and the rear plane of the cell. International patent application WO 02/16083 shows another mechanical structure in which the module is pinched by way of two elastic seals.
When a rear polarizer 4 of the type that is not hardened to storage in damp heat is used, the polarizer on plate assemblage is sensitive to dampness. For these reasons, a rear encapsulation structure is provided, configured to make this assemblage leaktight right around the whole cell. This encapsulation structure thus comprises, in addition to the inner frame 10.b and the spacer 12, a transparent slide of substantially the same dimension as the plate, typically a glass slide 13, combined with a dual-face adhesive frame 14 placed between the plate 1 and this glass slide 13: the plate, the slide and the adhesive frame form a leaktight cavity in which the polarizer is enclosed: under these conditions, the rear polarizer 4 is set back by a width d1 from the edge of the plate 13 and the glass slide 13, that is to say its surface is substantially smaller than that of the plate and of the slide.
The adhesive frame used is of VHB (“Very High Bonding”) type, that is to say highly adhesive. It is disposed at the plate and/or slide edge, and has a width d2 that is smaller than the setback d1. It has a thickness e2 which ensures a spacing of a few hundred micrometers between the glass slide 13 and the polarizer 4. This spacing is necessary so as to avoid contacts between the two opposing planes 4 and 13, which contacts would have as undesirable optical effects, the formation of interference fringes. The spacer 12 is pinched between the inner frame 10.b and the rear face of the glass slide 13. It has substantially the same width as the adhesive frame.
Depending on the purpose of the product and the optical qualities sought, it may be necessary to integrate additional optical functions at the rear, such as for example a diffuser, or optical flux amplifier, or prepolarizer function, etc. This can be done easily in the configuration described, by inserting other additional optical films at the rear face of the polarizer, in the space 5 between the glass slide 13 and the polarizer 4. For example, a diffuser 200 is illustrated in FIG. 1. This diffuser is integrated in a floating manner,—that is to say it is not glued to either plane of the polarizer 4 or slide 13; it “floats” in the air gap between the polarizer 4 and the glass slide 13. The diffuser can also be laminated on the rear slide so as to prevent it from floating in the cavity and therefore from crinkling in a thermal environment.
A rear encapsulation such as this exhibits the necessary mechanical, optical and leaktightness conditions. Notably, leaktightness is ensured by the VHB adhesive frame: the term “frame” signifying that the adhesive is continuous over the whole of the perimeter of the cell.
However, it exhibits various drawbacks:                the VHB frame has a high manufacturing cost: indeed, the frame is made by removing the central material from a sheet of adhesive material: the whole of this central portion is lost.        the VHB frame uses the peripheral edge 1a of the plate 1, outside of the active optical zone Z of the cell. The width e2 of the VHB frame depends on the width of this zone. This characteristic prevents the use of certain off-the-shelf cheap LCD cells in which the plate and the backplate are cut flush with the active zone on two sides at least. This is an inconvenient limitation since it is increasingly sought, even in constraining applications of avionics type, to use off-the-shelf components termed COTS, to reduce costs. In other cells, the peripheral zone is very narrow, too narrow to make a VHB frame of sufficient width to guarantee leaktightness. Typically below about three millimeters, a VHB frame is not leaktight. To this difficulty is added those of making and manipulating an overly slender VHB frame.        the air gap in the space 5 between the polarizer 4 and the glass slide 13 gives rise to high sensitivity of the LCD screen to variations in atmospheric pressure. The greater the distance between the two planes 4 and 13, the greater is this sensitivity. This sensitivity to pressure variation is very inconvenient for LCD screens dedicated to avionic applications, with risks either of opening during large pressure variations, or of bonding of smooth interfaces (rear shim with polarizer or DBEF, diffuser with rear shim, etc.)        according to the screen's storage conditions, an optical film such as 200, placed at the rear of the cell in the space 5 between the two planes 4 and 12, may crinkle. At the optical level, crinkling results in non-uniformities of the LCD back-lighting and therefore degraded image quality.        repair, for example to replace the crinkled film, requires the dismantling of the mechanical structure.        
For these various reasons, a rear polarizer 4 of the type hardened to storage in damp heat is preferably used, thereby making it possible to circumvent the problems of leaktightness at the rear plate/polarizer interface, and therefore of the glass slide plus adhesive frame mechanical structure of FIG. 1: the hardened polarizer 4 can be made over the whole extent of the plate 1, as illustrated in FIG. 2. The plate and polarizer assembly can then be pinched directly between the outer frame 10.a and the inner frame 10.b, via the seal j at the front face and the spacer 12 at the rear face, since there is no problem of leaktightness at the plate 1 and rear polarizer 4 interface. The spacer 12 can bear directly against the rear face of the polarizer 4.
Depending on the purpose of the product and the optical qualities sought, or in order to perfect the optical properties of the screen, it may be necessary to integrate additional optical functions at the rear face, such as for example a diffusing function or optical flux amplifier. In the configuration of FIG. 2, the integration of an optical film 200 is done by overlaying the film against the rear polarizer 4. However, as the polarizer exhibits a smooth rear surface, this can only be done by using an optical film exhibiting a nonzero surface roughness, as taught in the U.S. Pat. No. 4,508,428. The necessary roughness is obtained by appropriate treatment of the surface, by any known technique, and for example, in the case of a diffuser made of PMMA (PolyMethyl MethAcrylate, a thermoplastic of the plexiglass type): molding of the part in a structured mold, embossing, chemical or physical etching, etc. Otherwise, if the optical film also exhibits a smooth surface, the assemblage would introduce cosmetic defects such as interference or Newton's rings.
The polarizer 4/diffuser 200 stack simplifies the encapsulation of the LCD module in the mechanical structure 10, since the assembly can be clamped, via the seal and the spacer, between the outer frame and the inner frame without any additional mechanical element, and without any adhesive frame, unlike the encapsulation illustrated in FIG. 1.
However, it has been possible to observe in practice that liquid manages to infiltrate by capillarity at the interface of the diffuser 200 and polarizer 4 planes, through the rim of the cell. Liquid can thus be trapped between the two planes of the contacting surfaces of the diffuser and of the polarizer, as illustrated in FIG. 3. This liquid trapped at various places o1, o2 then gives rise to defects of non-uniformity of illumination. Once trapped, the liquid cannot easily evaporate. Even drying the screen may leave traces of dried water and/or water residues, which will have the same unacceptable optical effects.
More generally, as it may be beneficial to integrate several optical films at the rear, each ensuring a particular optical functionality, the problem of the leaktightness of the interface arises each time.
Finally, the replacement of an optical film exhibiting a defect necessarily requires the dismantling of the mechanical structure, which is not desirable.